The present invention concerns a brush contact for a vehicle, in particular for a vehicle that runs on rails and that has a propulsion unit that is to be supplied with electric current, with a sliding element for the transferral of the electric current from a live conductor that is carrying an electric current into the vehicle, whereby the sliding element contains carbon in at least one part of its contact surface that is turned toward the live conductor.
Vehicles that are used for the conveyance of passengers, such as trains, underground trains, street cars and trolley buses-tap off the current for their electrical propulsion units from a live conductor by means of a sliding element. The live conductors can be overhead wires that run above the track or, on the other hand, they can be current rails that are arranged inside the track or on the side of the track, for example, a third rail. The sliding element must engage the live conductor as continuously as possible in order to ensure the reliable transfer of current while traveling, for which purpose a uniform force is required to hold the contact surface of the sliding element in engagement with the live conductor. Such problems occur when the movement of the sliding element along the live conductor takes place at high speed such as is the case with, for example, high speed trains that travel at speeds far in excess of 250 km/h. At these speeds, one can observe a certain lifting up of the sliding element from the live conductor, whereby this leads to spark formation and is accompanied by increased wear of the sliding element because of spark erosion. In order to avoid such lifting up, a high force that presses the components together must be applied and this is associated with a high expenditure of power, especially in relation to a high mass of the brush contact. Conventional brush contacts for such high speed trains are assembled from two sliding elements that are arranged to run parallel to one another on a supporting frame, whereby their longitudinal extension is aligned approximately vertically to the live conductor. Sliding strips, which consist of graphite, are attached to the supporting frame in U-shaped supporting profiles, which are made of aluminum. The supporting frame itself is connected to the vehicle via an arm-shaped seesaw-type mounting support.
The graphite material that is used for the sliding element in accordance with the prior art exhibits high wear. Abrasive wear is caused by frictional contact with the live conductor, whereas oxidative wear arises via the aforementioned formation of sparks during the short-term lifting up of the sliding element from the live conductor during travel. A further problem is the low strength of the graphite material which necessitates a profiled element in order to supportively mount the graphite strips. In this way, the problem is posed to the effect that, on the one hand, the graphite strip has to be sufficiently wide for stability reasons and, on the other hand, an adequate height of the graphite strip has to remain free in order to ensure sufficient contact over the graphite material up to a certain level of abrasion. The losses of graphite material that are due to wear are considerable, especially under extreme conditions such as, for example, in the case of high speeds with a corresponding evolution of heat and also under the influence of extreme coldness, for example, during winter operation, whereby the live conductor confronts the graphite with an extremely uneven surface as a result of ice formation. In addition, the supportive mounting of the graphite strips in a profiled support leads to shapes that are unfavorable in regard to air flow and that, on the one hand, possess a high air resistance and lead to the development of noise during operational conveyance, especially at high speeds of conveyance. Another problem shown by the conventional sliding elements is the different thermal expansion coefficients between the graphite strips and the profiled supports, which are made of aluminum. Time-consuming connection of these different components with corresponding cost consequences arise from this. In order to compensate for the occasional lifting up of the sliding element from the live conductor that leads to interruptions in the current, each brush contact is equipped with two sliding elements that, as already mentioned above, are connected to one another via a see-saw mounting device, so that, as a result, it is the intention to assure, in each case, that at least one sliding element restores contact with the live conductor.
A brush contact for a railroad locomotive is known from U.S. Pat. No. 4,339,021 and includes a sliding element comprising carbon-fiber-reinforced carbon.
In addition, a brush contact material is known from JP-Abstr. 63-140,050 in which 5-15% by weight of copper-treated carbon fibers are added to a mixture comprising metal powder and graphite powder and the resulting mixture is compacted and sintered after mixing these components. The carbon fibers preferably possess a length of 0.2-10 mm and a coating thickness in the range from 1-10 mm.
DE-A1-39 33 039 discloses a process for the production of oxidation-protected CFC composite materials that comprise a carbon matrix and reinforcing fibers comprising carbon. These composite materials are initially carbonized and, after this, metallic silicon is infiltrated in the liquid state and then reacts with the matrix carbon in the pore zones and also with the carbon fibers to give silicon carbide. In this way, one obtains composite materials with improved oxidation protection but reduced quasi-ductility since the fiber reinforcement is attacked extensively.
A comparable process to that disclosed in DE-A1 39 33 039 is known from DE-A1-41 36 880 for the production of an oxidationresistant component, especially for space travel.
An aerodynamically constructed sliding plate for a high speed brush contact is known from DE-B2-8 30 027. The sliding plate consists of Cu-carbon, CuCd or NiAg.
U.S. Pat. No. 4,339,021 discloses a brush contact for a vehicle, in particular for a vehicle that runs on rails and has a propulsion unit that is to be supplied with an electric current, with a sliding element for the transferral of electric current from a current-carrying live conductor into the vehicle. The sliding element has at least one current-transferral region that is turned toward the live conductor and includes a composite material of carbon-fiber-reinforced carbon. The object of the present invention is to further develop a brush contact for a vehicle, in particular for a vehicle that runs on rails, which significantly reduces wear phenomena and exhibits improved brush contact even under extreme operating conditions.
Proceeding from a brush contact as disclosed in U.S. Pat. No. 4,339,021, silicon carbide formed by infiltration of liquid silicon and reaction with carbon, and electrically highly conducting materials in the form of pins, fibers, foils or strips that run in its longitudinal direction in the flow direction of the current are incorporated in the current-transferral region. As a result of the use of a composite material based on carbon-fiber-reinforced carbon, a higher abrasion resistance is achieved in comparison to graphite, especially under extreme conditions for traveling. In contrast to graphite, the weight of the sliding element and thus that of the brush contact is reduced as a result of this material. Because of the significantly lower mass, lower forces for pressing the components together can be applied in order to press the sliding element against the live conductor. Because of the lower dynamic forces, the forces that act on the sliding element as a result of unevenness along the live conductor are also reduced, so that the sliding element remains in uniform contact with the live conductor even at high speeds. In addition, it has been found that such a sliding element possesses high inherent stability so that expensive profiled elements to supportively mount and guide the sliding element are not required. Rather, such a composite material based on carbon-fiber-reinforced carbon can be equipped with attachment devices only on its lower side so that, in addition to a reduction in weight, the further advantage is achieved that the total height of the sliding element is available for brush contact so that a relatively great height is available for abrasion and a long service life results from this.
It has also been found that the very high temperatures that result from the sliding contact between the sliding element and the live conductor at high speeds, as these arise, for example, in the case of high speed trains, likewise have virtually no effect relative to increased abrasion.
Layer structures comprising silicon carbide are preferably incorporated in the carbon-fiber-reinforced carbon matrix that forms the basic structure of the sliding element. Such layer structures of silicon carbide increase the abrasion resistance of the sliding element. Such layer structures are thin and increase the total strength of the composite material as well as its abrasion resistance. Silicon carbide also excels in combination with the carbon-fiber-reinforced carbon matrix by virtue of the feature that it forms silicon carbide with the carbon from the carbon matrix via infiltration of silicon into the carbon matrix, as a result of which a high strength composite is produced. In addition, the oxidation resistance and thus the time of usage of the sliding element is increased by the silicon carbide.
In order to increase the electrical conductivity of the sliding element in a controlled manner while influencing the characteristics of the sliding element with regard to its abrasion resistance and its weight as little as possible, electrically highly conducting materials are incorporated in the carbon-fiber-reinforced Carbon element. These highly conducting materials preferably consist of very small components, preferably in the form of thin wires or fibers or small metal strips such as foils. Graphite components are also suitable, whereby graphite components are to be preferred when the conductivity of the graphite is adequate for the requirements of current conduction, whereas metal components are preferably added when the electrical properties of the sliding element have to be improved still further or when increased wear protection is required.
These components, which increase the electrical conductivity, are preferably oriented in such a way in terms of their largest dimension that they run vertically to the contact surface of the sliding element or, as the case may be, vertically to the surface of the live conductor. A preferred procedure in order to incorporate metal components into the sliding element is one in which drilled holes are provided in the C/C-SiC component, which is produced as indicated above, and run vertically to the later contact surface of the sliding element, whereby the holes are filled up with, for example, liquid copper. In this connection, the C/C-SiC component can be inserted into a mold and, simultaneously within the framework of filling the pores or passageway openings, a contact plate is cast on the brush contact side that lies opposite the contact surface, whereby the contact plate connects the individual holes or, as the case may be, copper (metal) pins to one another. The pores can also be formed by appropriate spacers that are inserted into the CFK green compact and are later removed. The distribution can be adjusted via the infiltration technique that was mentioned above by way of the feature, for example, that the C/C-component is infiltrated with different components from both sides. In addition, it is also possible to fill up the residual pores of the C/C-SiC element with, for example, molten Cu by means of liquid phase infiltration.
In addition to silicon carbide that increases the oxidation resistance, use can also be made of further wear-reducing components in the carbon-fiber-reinforced carbon matrix or, as the case may be, additives, that increase the wear resistance, in order to increase the operating life of such a sliding element. Titanium carbide and/or zirconium carbide are especially suitable for this purpose.
Preferably, a CFK green compact (carbon-fiber-reinforced polymer) is initially prepared in the final geometry of the brush contact. After this, the polymer matrix is transformed into carbon by means of pyrolysis so that a porous C/C (carbon-fiber-reinforced) component is produced. The voids that remain behind in the C/C component after pyrolysis are filled up with liquid metal (liquid phase infiltration). The molten metal, in the form of liquid silicon, reacts with the carbon matrix and also with the carbon fibers to give silicon carbide.
The addition of titanium carbide and/or zirconium carbide already takes place within the framework of the assembly of the CFK green compact, whereby the position of the addition can be varied, as a result of which a C/C-SiC component can be assembled with a gradient-based distribution of the additive substances. This addition can take place in powder form by being scattered, on the one hand, into the carbon-fiber fabric of the CFK green compact or in the form of an addition of the resin (polymer) during the assembly of the CFK green compact by means of a lamination process or an injection process. In each case, the inclusion of these components in the assembly of the CFK green compact is a preferred procedure, since optimum adjustment is possible even in the narrowly delineated zones of the titanium carbide distribution or the zirconium carbide distribution.
In order to achieve an optimum balance between a high electrical conductivity of the sliding element, high strength, good oxidation resistance and low weight, the proportion of silicon carbide and the added wear-reducing components should amount to between 15% and 50% based on the mass of the sliding element. The proportion of free, unbound silicon should amount to between 0% and 10% of the mass of the component; the range between 15% and 50% is applicable when the brush contact contains only silicon carbide in order to increase its abrasion resistance and oxidation resistance.
Components that increase the electrical conductivity can be introduced into the sliding elements in such a way that their proportion by volume in the individual layers varies proceeding from the contact surface to the opposite current-tapping surface, whereby the highest proportion of these components should lie in the region of the surface layer that is turned toward the live conductor. As a result of this, the conduction of current in the region of the surface layer and the oxidation resistance of the surface layer are increased, while the region of the brush contact side that, in the final analysis, is not subject to abrasion, even in the case of a worn-out sliding element, can be optimized in regard to its stability.
The electrically highly conducting materials, that are used in the form of fibers, pins and/or strips, should possess a cross-sectional dimension in the range from 0.1 mm to 0.5 mm (foil) and from 1 mm to 5 mm (strips and pins). Electrically highly conducting materials such as copper are introduced into the sliding elements in a defined manner.
The sliding element can be subdivided into individual zones by the components that increase electrical conductivity, whereby the zones or surfaces of increased conductivity run vertically to the live conductor or, as the case may be, the contact surface, and preferably run vertically to the sliding direction as has already been explained above. The inclusion of such zones of increased electrical conductivity preferably involves inclusion of components with volumes that are large in contrast to the embedded wires, sheets or foils. This has the advantage that the structure can be assembled by means of simple production methods, for example connecting techniques, with only a few individual parts that can be readily handled. In this connection, graphite zones with carbon-fiber-reinforced carbon zones, which have preferably been filled with silicon carbide, can alternate with each other to maintain good current conduction on the one hand and, on the other hand, in order to positively influence the wear characteristics.
The zones of increased electrical conductivity should have a thickness of 1 mm to 5 mm and the average distance from zone to zone should lie in the range from 5 mm to 10 mm, i.e. in the case of a sliding element, which can have a width of approximately 40 mm, two to three such zones of increased electrical conductivity are provided.
Since the brush contacts in accordance with the invention excel by virtue of sliding elements that can be assembled in a self-supporting mode of construction, it is possible to provide these sliding elements with a profile that is favorable in terms of aerodynamics. In conjunction with brush contacts for high speed trains, this ensures the low evolution of noise and, in addition, the turbulence in the area of the sliding element that has a negative effect on implementing uniform contact along the sliding wire is reduced. Irrespective of the supporting profiles that are required in accordance with the prior art, the possibility also exists of providing the sliding elements with a profile such that they experience increased pressing of their contact surface onto the live conductor at increasing speeds. A supporting cross-sectional profile is suitable for this purpose, which, for example, possesses a more marked curvature toward the live conductor, i.e. on the contact surface, than at the opposite current-tapping side, so that a negative pressure is produced at the live conductor. Because of the high material strength, it is also possible to construct the sliding element in the form of a hollow profile and this leads to a further reduction in mass together with the advantages that are associated therewith. A technique in which a carbon-fiber-reinforced carbon composite material is assembled in a green state, is again suitable for the preparation of such a profiled or hollow sliding element, whereby the composite material is then reinforced by infiltration with liquid silicon and a subsequent heat treatment with the formation of layer structures comprising silicon carbide. As a result, a high-strength sliding element is obtained with a final geometry that does not have to be reworked, although it can easily be optimized with respect to the desired geometries in the green state of the element.
Since vehicles, especially high speed trains, are not unambiguously defined in terms of their direction of travel, it is particularly advantageous--especially in combination with profiles of the sliding element that are favorable in terms of aerodynamics--to mount these in a manner that permits rotation about an axis that runs vertically to the contact surface and to the live conductor, in order to rotate this profile of the sliding element in accordance with the direction of travel. This can take place by means of an adjusting drive system, e.g. by means of an electric servomotor.
Because of the advantages in regard to low weight, good contact properties and the resultant smooth-running contact, it is possible to operate with a single sliding element on each brush contacts, in contrast to the prior art that utilizes two such sliding elements.